1. Field of the Invention
This invention relates to the production of purified hydrogen-rich gas from a synthesis gas mixture containing CO and acid gas contaminants including CO.sub.2, H.sub.2 S and COS.
2. Description of the Prior Art
Numerous methods for removal of acid gas from gas mixtures containing the same are well known in the art and in commercial practice. Included among the known processes for acid gas removal from gaseous streams are those employing physical absorption of CO.sub.2 and/or H.sub.2 S as distinguished from other processes involving chemical reaction. The physical processes are particularly preferred when the feed gas to be treated is available at high pressure and contains relatively large quantities of acid gas constituents and selective separation is desired. Numerous and diverse organic solvents have been suggested or utilized for the desired absorption. Included among the solvents used in known commercial process are methanol, employed in the Rectisol process (U.S. Pat. No. 2,863,527); N-methyl-2-pyrrolidone, used in the Purisol process (U.S. Pat. No. 3,505,784); propylene carbonate, used in the Fluor Solvent process (U.S. Pat. No. 2,926,751); and dimethyl ethers of polyethylene glycol, used in the Selexol process (U.S. Pat. Nos. 2,649,166; 3,362,133). Other proposed solvents include: N-substituted morpholine (U.S. Pat. No. 3,773,896); hexafluoroisopropyl alcohol (U.S. Pat. No. 3,339,342); dimethyl formamide or dimethyl sulfoxide (U.S. Pat. No. 3,676,356); among other solvents suggested as physical absorbents for acid gas are: tri-n-butyl phosphate, methyl cyanoacetate, glutanitrile, trimethylene cyanohydrin, N-formyl morpholine.
In addition to the many different types of absorption solvents heretofore used or proposed for use in desulfurization and CO.sub.2 removal from gas mixtures, a variety of differences in operation techniques and process conditions appear in the patented art and published technical literature. The more widely adopted systems, however, in general, follow an operational sequence that may be characterized as conventional hereinbelow described.
In these conventional processes for desulfurization and removal of CO.sub.2 from gas mixtures, such as those obtained by partial oxidation of heavy oils or by gasification of coal, the presence of COS in the feed poses difficulties in desulfurization when physical solvent absorption systems are employed. In such conventional processes the feed gas is charged to an absorption column wherein it is contacted with the selected physical solvent for absorption of H.sub.2 S and COS. The thus desulfurized gas is subjected to a catalytic shift reaction with steam wherein the CO is converted to CO.sub.2 and hydrogen is obtained. The resulting gaseous effluent from the shift converter is treated with a selected suitable solvent for absorption of CO.sub.2 and the resulting gaseous effluent is sent to a methanation section for hydrogenation of residual CO and CO.sub.2, obtaining a hydrogen-rich gas product. The fat liquor from the desulfurizing absorber is stripped of contained H.sub.2 S and COS, providing a product gas from which sulfur values may be recovered in a Claus plant and the lean solvent is recycled for reuse in further treatment of feed gas. The fat solvent from the CO.sub.2 absorber is flashed to remove a portion of the CO.sub.2 therefrom, then stripped of residual CO.sub.2 with air or inert gas and the thus stripped liquid is recycled for reuse in the CO.sub.2 absorber column.
The utility requirements for the operation of such conventional processes are comparatively costly. In certain of these conventional processes solvent flows required for COS removal in desulfurization results in a dilute Claus gas (typically containing about 11-12 mole % H.sub.2 S) which is too dilute for processing in conventional Claus plants for recovery of sulfur values. Accordingly, special expensive Claus plants need to be used, which require high purity oxygen instead of air for burning a part of the H.sub.2 S to SO.sub.2 or a sulfur product recycle oxidation. In addition, such processes require special expensive Claus tail gas units.
Other of these conventional processes for desulfurization of feed gas mixtures, such as those employing methanol as solvent for the sulfur gas, have been designed to produce a Claus gas of sufficiently high H.sub.2 S content that can be charged to a conventional Claus gas system. These systems, however, need to make use of an extra column to concentrate the H.sub.2 S. Other conventional processes for desulfurization of gas mixtures obtain a Claus feed containing from about 20% to over 50% H.sub.2 S. The solvents generally employed in such processes, such as, for example, methanol, N-methyl pyrrolidone or dialkyl ethers of polyethylene glycol, are such that the solubility of H.sub.2 S therein is much greater than that of CO.sub.2, while the solubility of COS is intermediate of these. When COS is absent the desulfurization solvent flow rate is set for essentially complete H.sub.2 S removal and only a small fraction of the CO.sub.2 is coabsorbed, so that the desired concentration Claus feed is obtained. When COS is present, however, a substantially higher solvent flow rate is required to obtain complete absorption and desulfurization, with consequent increase in equipment costs and utility requirements. The coabsorption of CO.sub.2 is also increased by the higher solvent flow rate and deep flashing of the rich solvent must be utilized to obtain a satisfactory Claus feed containing a required minimum of about 20% H.sub.2 S. In addition to the foregoing drawbacks, the increased compression requirements for the flash gas further adds substantially higher capital investment in equipment and higher power costs.
The hereinabove described difficulties and other drawbacks of these earlier known processes for desulfurization of gas mixtures are largely avoided in accordance with the novel process of the present invention and the economics of the operation are favorably improved, as will hereinafter appear.